A typical image-forming apparatus such as a printer or a copier that uses electrophotographic, ionographic, or magnetographic technologies frequently uses powder toner development of an intermediate image created in the forming process. With any of these image-forming technologies, a supply of powder toner is stored in a toner reservoir from which it is delivered via a developer roll and metering blade to a photoconductor drum.
For the case of electrophotographic printing, a photoconducting drum is first electrostatically charged; the photoconductor drum is then exposed to the image light pattern, which selectively discharges regions on the previously charged drum; the photoconductor drum is developed by delivering electrostatically charged toner particles to the surface of the drum where the charged particles selectively adhere to appropriately charged regions; the electrostatically transferred toner image on the drum is transferred to the paper (or other carrier medium); the toner is thermally fused to the paper; and any residual toner is cleaned from the surface of the photoconductor drum prior to reinitiation of the process. Such process is applicable for write-black printers as well as write-white printers.
According to the above steps, the delivery of powder toner to the photoconductor drum is referred to as development. Two different development techniques utilize powder toner; namely, a dual component and a mono component development technique. The dual component technique was most commonly utilized prior to the advent of electrophotographic printers designed for personal and work station computer use. However, the technique is still found in high-end printers. This technique requires the use of toner particles and carrier beads which must be provided in a supply reservoir. The other technique, referred to as mono component development, is used almost exclusively for low-end printers because the use of carrier beads is not required. However, such systems utilize powder toner, which is usually provided in a replaceable toner/developer cartridge. Hence, powder toner is supplied via a toner reservoir.
According to either development technique, there is a need to sense the level of toner available within a toner reservoir in order to monitor and/or predict the level of available toner and the available printing life, respectively. One previously utilized technique of sensing available toner level on a printer has been implemented in the form of an antenna. A metal rod is positioned to run parallel with a development sleeve in the toner reservoir at a distance of about five millimeters. The metal rod couples with an electrical field that is generated by an alternating current induced electrical bias of the development sleeve. Associated circuitry is provided to sense the change in field strength resulting from decreases in toner level between the rod and the sleeve. Such a system proves relatively inexpensive, but is only capable of sensing toner at, or near, the end of life for a toner cartridge. Typically, such a system is only capable of sensing end of life for a toner cartridge when less than five percent of the toner still remains within the cartridge. Additionally, the antenna is required to remain adjacent, or near, the development sleeve or else signal strength is lost when the antenna is positioned any further away from the development sleeve.
An alternative technique for sensing toner level involves the use of emitter and detector pairs that have been configured to sense the presence of toner within a toner supply reservoir. Such a technique requires the use of a viewing window and a wiper, the wiper being used to frequently clean toner from the window. The emitter and detector pairs are used to detect the presence of toner via a window. However, the window and emitter/detector pairs are subject to toner contamination which can render them ineffective at sensing the presence of toner via a window. Additionally, optical sensing systems such as an emitter/detector pairs typically have a significant amount of noise in the detected signal as toner flow and clouding may affect their accuracy. Even furthermore, optical sensing systems require that the wiper stir and squeegee the toner adjacent the viewing window in order to keep the toner from accumulating in front of the viewing window. However, the construction of the necessary mechanical parts proves to be complicated and expensive.
Both of the above-mentioned sensing systems are capable of detecting the presence of toner. However, as toner capacity has increased and printers have been put on networks, the monitoring of available toner level has become an important consideration in the management of printer performance.